This invention relates to methods of fabricating nonvolatile memory devices and devices so fabricated, and more specifically to methods of fabricating phase change memory devices and devices so fabricated.
Phase change memory devices are widely used as nonvolatile memory devices in many consumer, commercial and other applications. Phase change memory devices are also referred to as Phase-change Random Access Memory (PRAM) devices. As is well known to those having skill in the art, a PRAM cell may comprise a phase change material, such as a chalcogenide material, between first and second electrodes. When heated and cooled under appropriate conditions, the phase change material, such as the chalcogenide material, adopts one of two stable but programmable phases: crystalline or amorphous. The resistance of the crystalline phase is relatively low, whereas the resistance of the amorphous phase is relatively high. The program state of the cell may therefore be determined by measuring its resistance. PRAM devices have been designed to store one bit per cell (single level) or multiple bits per cell (multilevel). The design, fabrication and operation of PRAM devices as described in this paragraph is well known to those having skill in the art, and need not be described further herein.
As noted above, the operation of a PRAM cell depends upon the phase change material adopting a stable programmable phase, so as to exhibit a given resistance. Unfortunately, however, it has been found that the resistance may drift over time, as noted, for example, in Street et al., “States in the Gap in Glassy Semiconductors”, Physical Review Letters, 35(19):1293-1296 (1975); Kastner et al., “Valence-Alternation Model for Localized Gap States in Lone-Pair Semiconductors” Physical Review Letters, 37(22):1504-1507 (1976); and lelmini et al., “Recovery and Drift Dynamics of Resistance and Threshold Voltages in Phase-Change Memories” 54(2):308-315 (2007). This drift in resistance over time can adversely impact the reliability and/or operability of the phase change memory device.